Smaller turbine engines

"Total pressure ratios with altitude" I have no idea what that even means. However, a turbine engine is normally aspirated. The Ng is the same at take off as at altitude. If the compressor is turning the same RPM and the blades are fixed then the air flow is less. Just second grade math. On a -61 (PAY3) you are limited to 750 HP at take off. This corresponds to approx 2000 foot pounds with a prop speed of 2000 RPM. At FL270 you will be down to about 1000 foot pounds because you are temp limited. You are temp limited because you don't have adequate air from the compressor. The only reason you can maintain max airframe limited horse power is because most turbine installations use flat rated horse power. Again the example of the -61 is flat rated at 750 HP (airframe limitations). The thermodynamic rating of the -61 is around 900 HP. Which means a 900 HP engine can maintain 750 horsepower up to the altitude where the air flow drops to a level you can no longer keep the turbine temp below red line. The 900 HP is only attainable at STP.
A turbocharged piston engine can maintain full rated HP (thermodynamic limit) up to its critical altitude by increasing the exhaust flow through the turbine thus spinning the turbo charger compressor faster resulting in more air up to the point that the turbo charger can produce no additional flow. This is the critical altitude of a turbo charged engine and is commonly referred to as being bootstrapped.
Turbines are normally aspirated.
Edit on the Ng: Ng is ALMOST the same. It will be slightly less since you are limited to the 750 flat rated horse power on take off.
 
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ronnieh said:
Turbine engines are normally aspirated also. They lose power with altitude just like the NA piston.
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Ronnie is correct. Turbines run on the Brayton cycle, so they are constantly doing all 4 of the "suck, squish, bang, blow" parts of combustion at once. The compressor is the "suck" and "squish" portion. The combustor is the "bang" portion (to some degree the turbine section as well since that's where the power is extracted), and the exhaust is the "blow" portion.

Many turbines are "flat rated" which would be like saying you can only run 20". That gives you constant power up to a certain altitude. But it's not because the engine can't produce more, it's because you're limiting what you use of it.
 
turbine section temps (material limitations) usually limit % thrust at altitude.
 
It has been a while Ted since I talked to you. Family doing well? I am not sure I still have your email address. Would you PM it to me?
 
But back to the topic. Here is the statistic I posted in the other thread:

"Federal Aviation Administration studies indicate that piston engines in aircraft have a failure rate, on average, of one every 3,200 flight hours while turbine engines have a failure rate of one per 375,000 flight hours. Accordingly, for every turbine engine experiencing a failure, 117 piston engines will have failed."

Given this remarkable disparity, I'm surprised there hasn't been more impetus to develop small, 200 HP turbines.
 
Those turbine numbers include Part 25 airliners, which are much, much more reliable than, say, a PT-6.
 
Jay Honeck said:
But back to the topic. Here is the statistic I posted in the other thread:

"Federal Aviation Administration studies indicate that piston engines in aircraft have a failure rate, on average, of one every 3,200 flight hours while turbine engines have a failure rate of one per 375,000 flight hours. Accordingly, for every turbine engine experiencing a failure, 117 piston engines will have failed."

Given this remarkable disparity, I'm surprised there hasn't been more impetus to develop small, 200 HP turbines.
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I suppose if you don't mind a 50 GPH 182, that would work....but jet fuel is still $4/gallon, so that's a helluva lot of fuel cost to burn. And it would give that 182 the range of a 152. Or less. Jet fuel is denser than gasoline.

And while turbines do last much longer, they cost a LOT more to overhaul.

Even on a 747, the fuel weight far exceeds the passenger weight for a maximum-range flight. Useful load will have to be designed in, and you'll need larger airplanes to do even basic things.
 
Checkout_my_Six said:
turbine section temps (material limitations) usually limit % thrust at altitude.
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You would not reach material limitations if you had adequate air flow. Material limitations are adequate for rated thermodynamic horse power IF you had enough air. But, since it is NA there is not enough air for cooling and horsepower. Dude.
 
Ted DuPuis said:
Those turbine numbers include Part 25 airliners, which are much, much more reliable than, say, a PT-6.
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So what's the ratio of fan engine failures to turboprop failures? I'm betting it wouldn't change the big picure in this discussion.
 
I don't know the answer to that in actual numbers. But the little engines like the PT6 and the GTE 331 will certainly have a higher failure rate than the large fan engines. 50% percent more, four times??? Ted may have a better feel for that.
 
MauleSkinner said:
So what's the ratio of fan engine failures to turboprop failures? I'm betting it wouldn't change the big picure in this discussion.
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If you are interested, the limit for the first 6 stages of compression in the T-56 is .125" deep and .125" wide. We'd snake a 1/8th welding rod into the compressor taped to a bore scope and take a picture. we were allowed to blend any thing we could not fit the welding rod into. If not it was a power section replacement.
 
ronnieh said:
You would not reach material limitations if you had adequate air flow. Material limitations are adequate for rated thermodynamic horse power IF you had enough air. But, since it is NA there is not enough air for cooling and horsepower. Dude.
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Dude.....are you gonna make me dust off my thermo book and derive the temp and pressure ratio equations? :D
 
MauleSkinner said:
So what's the ratio of fan engine failures to turboprop failures? I'm betting it wouldn't change the big picure in this discussion.
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I don't have an exact number, but you also can't compare just fans to turboprops since the requirements for the bigger engines increase (as you'd expect).

Nobody's saying that pistons are more reliable - they aren't. However the real world numbers for GA turboprops vs. pistons I think is more like 10:1.

Add in the fact that there simply isn't the money to support a small turboprop development, and we're stuck with pistons.
 
We have to realize that all, and I mean all, of the certified turboprop engines we today fly behind in GA, were certified in the 50's and 60's. Allison, RR, Honeywell, PT6 etc. Back then, there was no CNC and the turbine blades were made on manual lathes and mills by skilled workers. Today, when anyone can buy a 5-axis CNC mill and stick it in their basement, why aren't there any cheaper turbines around with better specs? Because there's no money in that. There's money in selling people a 50 year old already certified dinosaur where all the R&D costs are amortized, that's why.

There is nothing technically stopping a small turbine to be made. Only business and the "old boy network" is stopping it. This is the golden goose for them - why would they want to change that?
 
Price some PT6 power turbine blades. You will need to be setting down.
 
ronnieh said:
Price some PT6 power turbine blades. You will need to be setting down.
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Back when,, a rebuilt rotor for the T-56 compressor was $150,000 plus core
 
Seems like TFE731 turbine blades are about $5k each, somewhere around 84 on a wheel. The wheel costs lotsa $ too.
 
I'm not convinced that turbines would fair any better than piston in a typical <100 hour per year private airplane, corrosion is a problem with everything.
 
James331 said:
Less things to go wrong actually. At lest from a in flight failure standpoint.
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Yep, it's a pretty simply concept and has been used for decades. Fewer moving parts that a recip and overall a better concept IMO.
 
Jay Honeck said:
But back to the topic. Here is the statistic I posted in the other thread:

"Federal Aviation Administration studies indicate that piston engines in aircraft have a failure rate, on average, of one every 3,200 flight hours while turbine engines have a failure rate of one per 375,000 flight hours. Accordingly, for every turbine engine experiencing a failure, 117 piston engines will have failed."

Given this remarkable disparity, I'm surprised there hasn't been more impetus to develop small, 200 HP turbines.
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One failure in 3200 hours seems very very high. That would mean (on average) 2/3rds of all piston engines fail prior to a 2000 hr TBO? Something sounds off.
 
GlennAB1 said:
The PT6 family is known for its reliability with an in-flight shutdown rate of 1 per 333000 hours since 1963, 1 per 651,126 hours over 12 months in 2016.
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Anecdotally I've seen enough PT-6s fail that I don't believe those numbers to be accurate. I also don't believe the overall 6-figure numbers, again personal experience. But, most of these engines are on twin engine aircraft and I'm guessing that a lot of failures don't go reported.

Conversely, I haven't seen enough piston engine failures to believe the 1/3,200 piston numbers. Generally, the problem with piston numbers is a lack of reliable statistics on the actual hours in the fleet.
 
I know, I'm not saying overhaul has anything to do with it, but 2/3rds of engines do not cause their owners to have to make a dead stick landing before the next overhaul, I think.... (Just using OH period as a convenient measure of hours, that's all)
 
GlennAB1 said:
First time I've ever heard that. Normally, the term "naturally aspirated" is associated with a type of induction system for reciprocating engines.
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Well sort of. Let me try one more time. A normally aspirated engine (any engine) has no way of increasing the airflow above its wide open throttle at standard temperature and pressure. A boosted engine, turbo charged for example, has a device that can increase the air flow above what it would be without said device. A normally aspirated engine might be able to do 29 inches of manifold pressure at sea level. That 29 inches is the pressure above a perfect vacuum. As the engine goes up in altitude or hotter air or anything that decreases the number of molecules of oxygen per unit volume the manifold pressure will decrease. With a turbocharger the RPM of the turbocharger is increased by opening the wastegate allowing more exhaust gas to enter and spin the compressor faster. This faster spin compensates for the reduced air density. You can continue to do this up until the wastegate is wide open allowing no increase in compressor RPM. This is the critical altitude and above this altitude you will start losing power. This is the way a boosted engine works.
Now to the turbine. There is no device to increase the flow above what it is at STP. The compressor Ng at sea level is the same at 20,000 feet. Therefore at 20,000 feet the air is much thinner and with the compressor still turning the same RPM is must produce less volume of air.
On a typical turbo prop installation there is excess air on take off from moderate altitudes. The excess air is used for cooling not more horsepower. In my above example if we have an engine capable of a thermodynamic horsepower of 900 then there will be excess air at sea level to allow the 900 HP and not over temp the engine. Most installations are flat rated due to limitations by the airframe manufacturer. Thus you use partial power to take off because you are horsepower limited. If you are allowed to use only 750 horsepower due to airframe limitations then the 150 horsepower (900 - 750) can be used to maintain the 750 to some greater altitude. But, since the engine is NA it can not make additional air and the engine will temp out due to lack of air. In other words if you had an installation that allowed the full thermodynamic rating of a given turbine, then that horsepower would be reached at the same time as the engine temped out. Any decrease in air density (climbing in altitude) would result in a lower power or an over temp situation since there is no way to increase the air volume. There is no adjustability in the compressor. And there is no throttle body or any kind of butterfly system controlling air. What is not burned is used for cooling. This is the best I can do. If this does not explain it then just ignore and continue thinking that turbines are not NA.
 
If you're saying a turbine (jet or prop) has a higher, or even similar full failure rate to a piston, I question your experience behind these power plants.
 
ronnieh said:
Well sort of. Let me try one more time. A normally aspirated engine (any engine) has no way of increasing the airflow above its wide open throttle at standard temperature and pressure. A boosted engine, turbo charged for example, has a device that can increase the air flow above what it would be without said device. A normally aspirated engine might be able to do 29 inches of manifold pressure at sea level. That 29 inches is the pressure above a perfect vacuum. As the engine goes up in altitude or hotter air or anything that decreases the number of molecules of oxygen per unit volume the manifold pressure will decrease. With a turbocharger the RPM of the turbocharger is increased by opening the wastegate allowing more exhaust gas to enter and spin the compressor faster. This faster spin compensates for the reduced air density. You can continue to do this up until the wastegate is wide open allowing no increase in compressor RPM. This is the critical altitude and above this altitude you will start losing power. This is the way a boosted engine works.
Now to the turbine. There is no device to increase the flow above what it is at STP. The compressor Ng at sea level is the same at 20,000 feet. Therefore at 20,000 feet the air is much thinner and with the compressor still turning the same RPM is must produce less volume of air.
On a typical turbo prop installation there is excess air on take off from moderate altitudes. The excess air is used for cooling not more horsepower. In my above example if we have an engine capable of a thermodynamic horsepower of 900 then there will be excess air at sea level to allow the 900 HP and not over temp the engine. Most installations are flat rated due to limitations by the airframe manufacturer. Thus you use partial power to take off because you are horsepower limited. If you are allowed to use only 750 horsepower due to airframe limitations then the 150 horsepower (900 - 750) can be used to maintain the 750 to some greater altitude. But, since the engine is NA it can not make additional air and the engine will temp out due to lack of air. In other words if you had an installation that allowed the full thermodynamic rating of a given turbine, then that horsepower would be reached at the same time as the engine temped out. Any decrease in air density (climbing in altitude) would result in a lower power or an over temp situation since there is no way to increase the air volume. There is no adjustability in the compressor. And there is no throttle body or any kind of butterfly system controlling air. What is not burned is used for cooling. This is the best I can do. If this does not explain it then just ignore and continue thinking that turbines are not NA.
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Are you sure that's how a wastegate works?

And, if increasing the compressor speed of a turbocharger compensates for reduced air density why wouldn't increasing compressor speed of a gas turbine engine also compensate for reduced air density?
 
EdFred said:
SO what's the break even point for turbine vs piston?
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Strictly on fuel burn, for me it's 300 miles. I'm comparing my old 421B to my 425, using the differences in total fuel burn and difference in fuel cost on the same trip, it's about $30 more each way from Atlanta to Destin for me. Om longer trips the Conquest is cheaper on fuel, mainly due to the price difference of the fuel.
Maintenance can be all over the board, the 425 has been more expensive to maintain, maybe 40-50% more, but I had a very clean 421 that was very reliable!
 
GlennAB1 said:
Are you sure that's how a wastegate works?

And, if increasing the compressor speed of a turbocharger compensates for reduced air density why wouldn't increasing compressor speed of a gas turbine engine also compensate for reduced air density?
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Sigh, you have pretty much beat me down. I went back and looked at what I wrote to make sure it was in English and that all the words were spelled correctly. It appears it is. So, yes I am sure that is the way a typical general aviation piston engine turbocharger wastegate works. I was using the TIO540 J2B as my model. And as I clearly stated, Ng is not adjustable which makes your question"why wouldn't increasing compressor speed of a gas turbine engine compensate for reduced air density" non sensical. I can not explain it any better, believe what you want to.
 
Wastegates function to regulate the power output of the turbocharger system. By controlling exhaust airflow, aircraft wastegates manage the turbine speed, and therefore the compressor housing intake.

This is a quote from the link you posted. What is confusing you. Perhaps you are just having fun?

N1 N2 are specifically for fan jets and not turbo props. I am through. Believe what you like.
 
ronnieh said:
Wastegates function to regulate the power output of the turbocharger system. By controlling exhaust airflow, aircraft wastegates manage the turbine speed, and therefore the compressor housing intake.
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You had your wastegate upside down in your comment. Wastegate is opened to allow exhaust gases to bypass the turbine. Wastegate fully closed means high exhaust energy through the turbine, and fully open means as little energy as possible through the turbine.
 
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Nope, wastegate opens to allow exhaust gas to bypass the turbine. No exhaust gas in the compressor. But I did type open when I meant close. The principle is the same. More exhaust gas into the turbine spins the compressor faster compensating for the thin air in a turbocharger. Compressor speed does not change in a turboprop once maximum thermodynamic horsepower is reached which will be at STP. Good night all.
 
ronnieh said:
Nope, wastegate opens to allow exhaust gas to bypass the turbine. No exhaust gas in the compressor. But I did type open when I meant close. The principle is the same. More exhaust gas into the turbine spins the compressor faster compensating for the thin air in a turbocharger. Compressor speed does not change in a turboprop once maximum thermodynamic horsepower is reached which will be at STP. Good night all.
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And for some reason, I typed compressor when I meant turbine. Been a long day...
 
Quite a few turbo installations in aircraft have fixed wastegates, including the airplane I'm currently flying...

Just to flog this dead horse. Haha.
 
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